VMs on macOS
I don't think we can count that, as it's a VM and not really running on the Mac - but it'd work in WSL and that's not really a VM as far as I'm aware. We can count WSL - I think. Running it in a VM isn't really running it in the OS.
But, even if we cheat and use VMs:
LOL There are an unreal number of OSes out there. I'll give you $1000 via PayPal if you can make AppImages run on all of them. You don't even have to bet me. It's just a free gift if you can get 'em to run on all of the OSes out there.
Though, I suppose you will have to publish your work. That way it's useful for other people.

